The present invention relates, in general, to evanescent mode radiators and, more particularly, to circularly polarized evanescent mode radiators.
Evanescent waveguide mode radiators (EMRs) may be used in phased array antennas because their small electrical size permits close inter-element spacing. Close inter-element spacing is necessary to prevent grating lobes when the antenna beam is steered to large off-broadside scan angles.
EMRs are also useful in applications where low radar cross-section (RCS) of the antenna is desirable since they emulate a low impedance surface at all frequencies below and an octave above a narrow, controllable operating band.
Present EMRs are inherently linearly polarized whereas circular polarization is often required. Presently, circular polarization is provided by exciting two orthogonal modes in a square waveguide cross-section through an external 90 degree hybrid, or by adding a meander-line polarizer across the aperture of the antenna. The former is undesirable in phased array applications where space, part-count, and weight are at a premium. The latter is undesirable since it adds to the thickness of the array and increases the loss and RCS of the antenna. It is also a technically difficult problem to maintain low side lobe levels at wide scan angles when a meander-line polarizer is used to generate circular polarization from a linearly polarized array.
Use of an evanescent mode waveguide as a resonant circuit element is known in the art. See Craven, "Waveguide Below Cutoff: A New Type of Microwave Integrated Circuit", 13 Microwave Journal, Vol. 13 No. 8, p. 51 (August 1970). The use of an evanescent mode waveguide as a linearly polarized antenna element is described in chapter 7 of EVANESCENT MODE MICROWAVE COMPONENTS, Craven & Skedd, (Artech House, 1987).
Circularly polarized evanescent mode radiators are utilized to communicate with a receiving antenna which is not stationary, or for a stationary receiving antenna to communicate with a moving transmitting antenna. This type of radiator would be beneficial to communicate with such items as aircraft and the like. In addition, this can be used to communicate between points where the orientation of one of the antennas may be unknown. Circular polarization can be generated by imposing a 90 degree phase shift between spatially-orthogonal, linearly polarized components.
Since space, or volume, is generally a concern in the design of microwave communication devices, a continuing object in the area of microwave radiators is to provide more compact and lighter weight radiators.
Accordingly, it is an object of the present invention to provide a circularly polarized evanescent mode radiator which overcomes the above deficiencies.
A further object of the present invention is to provide a circularly polarized evanescent mode radiator which is more compact.
Another object of the present invention is to provide a circularly polarized evanescent mode radiator which has a low radar cross-section.
Still another object of the present invention is to provide a circularly polarized evanescent mode radiator which does not require external components such as a 90 degree hybrid.